A swath printer is a raster or matrix type printer that is capable of printing a plurality of rows of dots in a single scan of a movable print carriage across a print media. The possible locations for dots that can be printed by a raster printer can be represented by an array or grid of pixels or square areas arranged in a rectilinear array of rows and columns wherein the centre to centre distance or dot pitch between pixels is determined by the resolution of the printer. For example, if a printer is capable of printing 300 dots per inch (dpi), the dot pitch of the pixel array would be 1/300th of an inch.
The print carriage of a swath printer typically includes a plurality of printing elements (e.g., ink jet nozzles) displaced relative to each other in the media motion direction which allows printing of a plurality of rows of dots. Typically, the separation between the printing elements in the media scan direction corresponds to the dot pitch for the finest raster row resolution that can be printed by the printer in a single carriage scan (e.g., 1/300th of an inch for 300 dot per inch (dpi) resolution). The printing elements of a swath printer are commonly implemented in a printhead such as a thermal ink jet printhead that is integral to a replaceable thermal ink jet printhead cartridge.
The quality of the printed images produced by a raster printer depends to large degree on the resolution of the printer. Higher or finer resolution, wherein the printed dots are more closely spaced, provides for higher quality images. To increase the resolution and print quality, the ink jet nozzles must be placed closer together. However, the dense packing of printing elements in a printing cartridge causes problems in providing electrical connections to the printing elements and in dispersing heat away from the printing elements. These problems are accentuated when the printing elements are activated or fired simultaneously.
U.S. Pat. No. 5,604,519 describes an ink jet printhead in which the ink jet nozzles are grouped or organised into fourteen primitives. The ink jet nozzles in each primitive are positioned in close proximity to each other and are activated individually (one at a time) according to a timing sequence or cycle. This sequential activation permits sharing of power supply lines and helps to overcome problems associated with firing the nozzles simultaneously.
FIG. 1 shows a simplified version of one of the primitives described in U.S. Pat. No. 5,604,519. The diagram illustrates the layout of eight nozzles in the primitive as viewed from an above the nozzles. The printing elements of the primitive, labelled N1 to N8, are scanned over the print media in a horizontal direction indicated by arrow A. The first printing element N1 is activated for printing by applying, for a predetermined period of time, an electrical power source at a "primitive select" terminal associated with that printing element. Following activation of the first printing element N1, the second printing element N2 is activated for printing by applying an electrical power source at a terminal associated with the second printing element N2. Subsequently, the third, fourth, fifth, sixth, seventh, and eighth printing elements N3, N4, N5, N6, N7, N8 are activated in sequence. During activation of the printing elements, any one of the activated printing element may be selectively fired by applying, at the appropriate moment, a control voltage at an "address select" terminal associated with that printing element.
Each printing element N2 to N8 is spatially offset in the horizontal direction from the preceding numbered printing element, i.e. N2 is offset from N1, N3 is offset from N2, and so on. The size of the offset, indicated as d in FIG. 1, is the same for each printing element N2 to N8. Thus, although the printing elements N1 to N8 are activated at eight different times, the offsets allow ink drops that are selectively fired from the activated printing elements to be placed at the same horizontal position on the print media. As the printing elements are also evenly spaced from each other in the vertical direction, the result of printing at the same horizontal position effectively creates a vertical printing column on the print media. The precise activation timing for each printing element N1 to N8 is based on a timing sequence that is specific to the size of the offset d and the scanning speed of the primitive. Scanning in the horizontal direction indicated by arrow B is also possible, whereby the sequence of activating the printing elements is reversed.
In order for the printing elements to print in a rectilinear array of rows and columns, the sequence of activating the printing elements is repeated or cycled during a single scan. This cycling produces a series of vertical printing columns whose separation in the horizontal direction is equal to the resolution of the printer in that direction. In this way, the printer is able to produce a standard raster or matrix print output on the print media. FIG. 2 shows a rectilinear array of rows and columns on a print media. The X's in FIG. 2 indicate the pixels in the array which the printer can selectively print to in a single scan.
Throughput, i.e. the speed of printing, is an important consideration in the design of a printer. In general, users prefer printers which can print faster. However, the higher the resolution of a printer, the more difficult it is to operate with a high throughput. The maximum scanning speed of a printhead is limited by the highest firing frequency of the printing elements and the separation of the vertical printing columns. The scanning speed may be calculated by dividing the distance travelled between firing a particular printing element (vertical column spacing) by the time interval between firing (reciprocal of firing frequency) or by dividing the firing frequency of the printing elements by the printing resolution in the horizontal direction (in dots-per-inch). An increase in the resolution of a printer results in a decrease in the separation of the vertical printing columns. This decrease, for a particular firing frequency of the printing elements, causes the scanning speed to decrease accordingly. For example, the printhead in U.S. Pat. No. 5,604,519, which has a relatively high resolution of 600 dots-per-inch and a maximum firing frequency of 12 kHz, is able to scan at 20 inches-per-second (ips).
This limitation on the maximum printing speed occurs in existing printers and is overcome by allowing the printer to operate in a mode having a lower resolution. In this mode, commonly referred to as a draft mode, the printer prints in alternate vertical printing columns, i.e. in every other vertical printing column the printing elements are not activated. The HP Deskjet 850, available from Hewlett-Packard, USA, is operable in a draft mode similar to that described above. FIG. 3 shows a rectilinear array of rows and columns on a print media which a printer operating in this draft mode can print to in a single scan. The X's in FIG. 3 indicate the pixels in the array which the printer can selectively print to in the above-mentioned draft mode. In FIG. 3, it can be seen that the resolution of the printer in the direction of the scan axis is halved, which in turn leads to doubling of the distance travelled between successive firings of the printing elements. Consequently, a printer operating in this draft mode has the potentially to print at speeds of up to twice the maximum speed of the standard higher resolution mode.
However, the applicant has found, in practice, that for printers having printing elements which are sequentially activated, the maximum scanning speed in the draft mode is less than twice the maximum speed in the standard mode. This shortfall is the result of a second limitation on the maximum scanning speed caused by the finite firing time of the printing elements. This firing time is the time period that each printing element is activated for during the sequence of activating the printing elements. For a thermal ink jet type of printing element, there is an associated minimum firing time which is determined by the minimum time required for ink in the element to be thermally excited to a vaporised state.
For the primitive shown in FIG. 1, the speed of scanning is equal to the offset distance, d, divided by the time period between firing one of the printing elements and firing the next element (the so-called stagger time). For a particular offset distance, d, the speed of scanning may be increased so as to minimise the stagger time. However, the stagger time cannot be made less than the minimum firing time of the printing elements, otherwise the printing elements cannot be activated individually (one at a time). The maximum scanning speed is thus limited to the offset distance, d, divided by the minimum firing time of the printing elements.
The second limitation is a drawback for printers having printing elements which are sequentially activated as it limits the potential scanning speeds available in a draft mode. Printers having printing elements which are activated simultaneously do not experience the firing time limitation and can thus achieve higher scanning speeds in a draft mode. Printers having printing elements which are sequentially activated are therefore at a competitive disadvantage.